The present invention relates to a nonlinear distortion correction circuit for suppressing the generation of a nonlinear distortion which is attributable to an incomplete input-output linearity characteristic in an amplifier using a transistor or an electronic tube. More particularly, the invention pertains to a feed forward distortion correction circuit.
As input-output nonlinearity correction means for amplifiers which works well in microwave and other high frequency bands, there has been known a feed forward distortion correction circuit disclosed in U.S. Pat. No. 1,686,792 issued to H. S. Black on Oct. 9, 1928. This correction circuit is referred to also as a feed forward amplifier.
The conventional feed forward distortion correction circuit is, as depicted in FIG. 1, composed basically of two loops: one is a distortion detecting loop 16 and the other is a distortion removing loop 17.
The distortion detecting loop 16 comprises a signal amplifying path 12, a linear signal path 13, a power splitter 3 for splitting input signal power for delivery to the two paths 12 and 13, and a power splitter/combiner for receiving outputs from the paths 12 and 13 and delivering an amplified signal and a distortion signal. The distortion removing loop 17 comprises a linear signal path 14, a distortion injection path 15, and a power combiner 5 for combining output powers from the two paths 14 and 15. The signal amplifying path 12 includes a main amplifier 6, and the linear signal path 13 includes a variable attenuator 8, a variable delay line 9 and a phase shifter 18. The linear signal path 14 is formed by a transmission line, and the distortion injection path 15 includes a variable attenuator 10, a variable delay line 11, a phase shifter 18' and an auxiliary amplifier 7. The signal amplifying path 12 and the linear signal path 13 of the distortion detecting loop 16 are connected via the power splitter/combiner 4 to the linear signal path 14 and the distortion injection path 15 of the distortion removing loop 17.
Even if one or both of the variable attenuator 8 and the variable delay line 9 are provided in the signal amplifying path 12, it does not make much difference in terms of circuit characteristics. Similarly, one or both of the variable attenuator 10 and the variable delay line 11 may be provided in the linear signal path 14. The phase shifters 18 and 18' each for phase inversion use may also be provided in the paths 12 and 14, respectively. The power splitter 3 is composed of a hybrid circuit. The power splitter/combiner 4 and the power combiner 5 are each formed by a hybrid directional coupler or similar circuit and can be regarded as a simple loss-free linear element.
Now, the operation of the above prior art feed forward amplifier will be described. An input signal applied to an input terminal 1 is applied first to the power splitter 3, wherein it is split into two signals having the same amplitude or an appropriate level difference, and then the signals are provided on the signal amplifying path 12 and the linear signal path 13, respectively. These signals are power-split and combined by the power splitter/combiner 4. The power splitter/combiner 4 is formed by such a directional coupler as that whose transmission losses between ports 4-1 and 4-3 and between ports 4-2 and 4-4 are negligibly small, say, about 0.1 dB, but those between ports 4-1 and 4-4 and between ports 4-2 and 4-3 are as large as 20 dB, for instance.
The variable attenuator 8 and the variable delay line 9 are adjusted so that the signal component which is applied from the signal amplifying path 12 to the distortion injection path 15 via the power splitter/combiner 4 and the signal component which is applied from the linear signal path 13 to the distortion injection path 15 via the power splitter/combiner 4 are equal in amplitude and delay but opposite in phase. The phase shifter 18 for phase inversion use may be implemented by inserting in the path 12 or 13 a circulator having its one port 19 terminated with a short-circuit as shown in FIG. 2. It is also possible to obtain the required phase inversion function, without providing the phase shifter 18, by suitably setting the phase shift amounts between the input and output ports of the power splitter 3 or power splitter/combiner 4, or forming the main amplifier 6 as a phase inverting amplifier.
Since the distortion detecting loop 16 is constructed as mentioned above, the difference component between the two signal components delivered from the two paths 12 and 13 is detected as the output of the port 4-4 of the power splitter/combiner 4. This difference component is the overall distortion component produced by the main amplifier 6, and loop 16 is called the distortion detecting loop after this function.
The distortion component derived at the port 4-4 of the power splitter/combiner 4 is adjusted in amplitude by the variable attenuator 10, adjusted in delay amount by the variable delay line 11, reversed in phase by the phase shifter 18', and amplified by the auxiliary amplifier 7, thereafter being provided to the power combiner 5. The amplified signal component derived at the port 4-3 of the power splitter/combiner 4 is applied via the linear signal path 14 to a port 5-1 of the power combiner 5. The power combiner 5 is formed by a directional coupler. The transmission loss between the ports 5-1 and 5-2 is negligibly small, say, 0.1 dB, whereas the transmission loss between ports 5-3 and 5-2 was as large as 20 dB, for instance. Accordingly, in order to cancel the distortion component contained in the amplified signal component which is input from the path 14 into the power combiner 5, it is necessary to increase the output of the auxiliary amplifier 7 to such an extent as to compensate for the transmission loss between the ports 5-3 and 5-2.
The variable attenuator 10 and the variable delay line 11 are adjusted so that the signal component having passed through the path 14 from the input port 4-1 of the power splitter/combiner 4 to the output port 5-2 of the power combiner 5 and the signal component having passed through the path 15 are equal in amplitude and delay but opposite in phase. In this instance, since the input signal to the path 15 is a distortion component produced by the main amplifier 6 and detected in the distortion detecting loop 16, the path 15 injects, at the power combiner 5, the distortion component into the output signal of the main amplifier 6 from the path 14 in the opposite phase but with the same amplitude with respect to the distortion component in the signal from the path 14, thus causing the distortion components to cancel each other at the output terminal 2 of the distortion correction circuit. The phase shifter 18' can also be formed by such a circulator as depicted in FIG. 2, but it is also possible to employ an arrangement in which the amounts of phase shift between the input and output ports-in the power splitter/combiner 4 or power combiner 5 are set to suitable values, or the auxiliary amplifier 7 is formed as a phase inverting amplifier, instead of using the phase shifter 18'.
The above is an ideal operation of the feed forward amplifier. Summing up its principle of operation, only the distortion component produced by the main amplifier 6 is detected in the distortion detecting loop 16 and is increased in level by the auxiliary amplifier 7, and then, in the distortion removing loop 17, it is reinjected into the main amplifier output in the opposite phase and with the same amplitude to thereby reduce the distortion. This implements an amplifier of excellent linearity.
In this case, it is necessary to minimize the transmission loss during transmission from the output terminal of the main amplifier 6 to the output terminal 2 via the power splitter/combiner 4, the path 14 and the power combiner 5 so as to avoid dropping of the output level of the feed forward amplifier. To meet this requirement, the power splitter/combiner 4 and the power combiner 5 are so constructed as to minimize the transmission loss between the ports 4-1 and 4-3 and the transmission loss between the ports 5-1 and 5-2. Since each of the power splitter/combiner 4 and the power combiner 5 operates as a loss-free circuit in its entirety, it is necessary to minimize the signal power which is delivered from the port 4-1 to 4-4 and the signal power which is delivered from the port 5-3 to 5-2.
This inevitably increases the transmission loss between the ports 4-1 and 4-4 and the transmission loss between the ports 5-3 and 5-2. In order that the transmission losses between the ports 4-1 and 4-3 and between the ports 5-1 and 5-2 may be held within 0.1 dB as mentioned previously, it is necessary that the transmission losses between the ports 4-1 and 4-4 and between 5-3 and 5-2 be larger than 20 dB. Granting that the transmission loss between the ports 4-1 and 4-4 is 20 dB, if the gain of the main amplifier 6 is set to 20 dB, the signal component from the port 4-1 to 4-4 will have a power level substantially comparable to that of the signal component which is input into the port 4-2 from the path 13 and delivered to the port 4-4. On the other hand, since the transmission loss between the ports 5-3 and 5-2 is more than 20 dB, the output signal level of the auxiliary amplifier 7 (which is used as a distortion correction signal) at the port 5-3 must be more than 20 dB above the distortion correction signal level which is needed to cancel the distortion component in the signal which is input into the power combiner 5 from the path 14.
FIGS. 3A, 3B, 3C and 3D show examples of signal spectra which occur at respective circuit points in the feed forward amplifier in the case where two signals of frequencies f.sub.1 and f.sub.2 and of the same amplitude were applied to the input terminal 1 under the above-mentioned design conditions.
FIG. 3A shows the output spectrum of the main amplifier 6. Frequency components f.sub.1 and f.sub.2 are the fundamental wave output components of linearly amplified input signals, 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1 are third intermodulation distortion components, and 3f.sub.1 -2f.sub.2 and 3f.sub.2 -2f.sub.1 are fifth intermodulation distortion components. Assuming that the output level of the main amplifier 6 is now in the vicinity of a saturated output, the level difference between the fundamental wave output component f.sub.1 or f.sub.2 and the third intermodulation distortion component 2f.sub.1 -f.sub.2 or 2f.sub.2 -f.sub.1 usually becomes 20 dB or below owing to the nonlinear characteristics of the amplifier. In other words, the level of the third intermodulation distortion component becomes close to the fundamental wave component.
FIG. 3B is the output spectrum of the distortion detecting loop 16, i.e. the output spectrum from the port 4-4, showing the state in which the fundamental wave components are sufficiently suppressed and distortion components are obtained.
FIG. 3C shows that, for example, in the case where the two third intermodulation distortion components in the output from the port 4-4 are regarded as input fundamental waves to the auxiliary amplifier 7, third intermodulation components 2(2f.sub.1 -f.sub.2)-(2f.sub.2 -f.sub.1)=5f.sub.1 -4f.sub.2 and 2(2f.sub.2 -f.sub.1)-(2f.sub.1 -f.sub.2)=5f.sub.2 -4f.sub.1 are newly developed by the auxiliary amplifier 7 in addition to the third intermodulation distortion components 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1 and the fifth intermodulation distortion components 3f.sub.1 -2f.sub.2 and 3f.sub.2 -f.sub.1 input into the auxiliary amplifier 7.
In the power combiner 5 of the distortion removing loop 17 these distortion components 2f.sub.1 -f.sub.2, 2f.sub.2 -f.sub.1, 3f.sub.1 -2f.sub.2 and 3f.sub.2 -2f.sub.1 shown in FIG. 3C are combined with the amplified signal provided from the path 14 thereby cancelling the distortion components depicted in FIG. 3A. In this instance, however, the third intermodulation distortion components 5f.sub.1 -4f.sub.2 and 5f.sub.2 -4f.sub.1 produced by the auxiliary amplifier 7 do not go down in level and remain as distortions of the the feed forward amplifier, as depicted in FIG. 3D. Incidentally, the output level of the auxiliary amplifier 7 (the amplifier distortion correction signal level) must be more than 20 dB above the signal level at the output terminal 2 as referred to previously. This means that if the level difference between the fundamental wave component and the third intermodulation distortion component in the main amplifier output is assumed to be 20 dB, the output level of the auxiliary amplifier 7 must be substantially equal to or higher than the output level of the main amplifier 6.
Accordingly, even if the auxiliary amplifier 7 has a maximum output power nearly equal to that of the main amplifier 6, the level difference between the fundamental wave components f.sub.1 and f.sub.2 in the output of the feed forward amplifier and the third intermodulation distortion components 5f.sub.1 -4f.sub.2 and 5f.sub.2 -4f.sub.1 attributable to the nonlinearity of the auxiliary amplifier 7 is 40 dB at a maximum, because the level difference between the assumed fundamental wave components 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1 and the third intermodulation distortion components 5f.sub.1 -4f.sub.2 and 5f.sub.2 -4f.sub.1 in the output of the auxiliary amplifier 7 is also about 20 dB as is the case with the main amplifier 6. The amount of distortion reduction thus achieved by the feed forward arrangement--evaluated in terms of the levels of the residual distortion in the cases where the auxiliary amplifier 7 is operated and is not operated--is only 20 dB at most.
Furthermore, when the maximum saturated output level of the auxiliary amplifier 7 is lower than that of the main amplifier 6, the level of the distortion correction signal from the auxiliary amplifier 7 becomes lower than the level necessary for complete cancellation of the third distortion components in the signal which is supplied from the main amplifier 6 to the power combiner 5 via the path 14, and consequently, the residual third distortion components in the signal which is derived at the output terminal 2 increase accordingly.
While the above has described the operation for correcting the third intermodulation distortion, the operation for the fifth intermodulation distortion is basically the same as described above. In FIG. 3B third intermodulation components which fall in the same frequencies as those of the fundamental waves are not shown for the sake of brevity, but in practice, the distortion components resulting from the nonlinear characteristics of the amplifier include, in addition to the third distortions of the frequency components 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1, third distortion components of the same frequencies as the fundamental frequencies f.sub.1 and f.sub.2 which are produced by the main amplifier 6, and these distortion components are also suppressed on the same principle as described above.
As described above, the prior art feed forward amplifier operates as an amplifier of more excellent linearity than in the case of using the main amplifier 6 alone, but it has the defect of necessitating, as the auxiliary amplifier 7, a power amplifier of a maximum output equal to or larger than that of the main amplifier 6 so as to achieve as large an amount of distortion reduction as 20 dB or more at an operating point where the output level is near saturation.